1. Field of the Invention
The present invention relates to organic electroluminescent devices.
2. Description of the Background Art
Organic electroluminescence devices (hereinafter referred to as organic EL devices) are expected as new self-light emitting devices. An organic EL device has a stacked layered structure that a carrier transport layer (an electron transport layer or a hole transport layer) and a luminescent layer are formed between a hole injection electrode and an electron injection electrode.
Electrode materials having a large work function such as gold or ITO (indium-tin oxide) are employed for the hole injection electrode, while those having a small work function such as Mg (magnesium) or Li (lithium) are employed for the electron injection electrode.
Organic materials are employed for the hole transport layer, the luminescent layer and the electron transport layer. Materials having the property of a p-type semiconductor are employed for the hole transport layer, while those having the property of an n-type semiconductor are employed for the electron transport layer. The luminescent layer is also composed of organic materials that have carrier transportability such as electron transportability or hole transportability and emit fluorescence or phosphorescence.
These hole injection electrode, hole transport layer, luminescent layer, electron transport layer and electron injection electrode are stacked in turn to form the organic EL device.
Each function layer such as the hole transport layer, the electron transport layer and the luminescent layer is constituted by a plurality of layers or omitted depending on the organic materials to be used.
In such an elementary structure as shown in Appl. Phys. Lett., Vol. 55, pp. 1489-1491 by Chihaya Adachi et al., for example, only two organic layers, which are a luminescent layer and an electron transport layer exist between a hole injection electrode and an electron injection electrode. This is because the luminescent layer composed of luminescent materials called NSD has excellent hole transportability and hence serves also as a hole transport layer.
Further, the elementary structure shown in Appl. Phys. Lett., Vol. 51, pp. 913-915 (1987) by C. W. Tang et al. is constituted by two organic layers, which are a hole transport layer and a luminescent layer. In this case, tris(8-hydroxyquinolinato)aluminum (hereinafter referred to as Alq) contained in the luminescent layer serves to both emit light and transport electrons.
On the other hand, the elementary structure shown in Appl. Phys. Lett., Vol. 69, pp. 2160-2162(1996) by S. A. Van Slyke et al. is constituted by three organic layers, which are a hole injection layer, a hole transport layer and a luminescent layer. In this case, the hole injection layer is composed of copper phthalocyanine, serving for the same function as the hole transport layer, which results in two hole transport layers existing in the entire device.
Thus, the number of the electron transport layer, hole transport layer and luminescent layer can freely be adjusted depending on the organic materials to be used.
A triplet luminescent material is considered as a promising material for a luminescent layer having high luminescent efficiency. The triplet luminescent material transits from a triplet excited state to a ground state to generate phosphorescence. An organic EL device including a luminescent layer of a triplet luminescent material has a device structure as follows.
A hole injection electrode (anode), a hole transport layer, a luminescent layer, a hole blocking layer, an electron transport layer and an electron injection electrode (cathode) are stacked in this order on a glass substrate. The luminescent layer includes 4,4′-bis(carbazol-9-yl)-biphenyl (hereinafter referred to as “CBP”) as a host material, and a triplet luminescent material as a luminescent dopant.
In this device structure, the hole blocking layer is used to prevent the electron transport layer from emitting light. More specifically, electrons are injected from the electron injection electrode into the electron transport layer, passed through the hole blocking layer, then injected into the luminescent layer and recombined with holes. Meanwhile, holes are injected from the hole injection electrode, passed through the hole transport layer, then injected into the luminescent layer, and recombined with electrons. In order to improve the recombination probability between holes and electrons in the luminescent layer, the holes must be prevented from penetrating through the hole blocking layer and being injected into the electron transport layer. Therefore, a highly stable hole blocking material having a high hole blocking characteristic is necessary as a material for the hole blocking layer.
As a conventional example, the use of Bathocuproine (hereinafter referred to as “BCP”) as a hole blocking material is suggested in M. A. Baldo, et al., Appl. Phys. Lett., 75. 4, 1999. The material has a molecular structure expressed by the following formula (6). Note that the formal name of BCP is 2,9-dimethyl-4,7-diphenyl-1, 10-phenanthroline. 
The BCP is however prone to crystallization with time after it is formed into a film. Therefore, the organic EL device produced using the BCP tends to suffer from current leakage, and stable light emission is not provided.